The invention pertains to a process for the application of a transparent metal oxide layer on a film, in which a metal is vaporized in a receiver and brought into contact in the vapor phase with oxygen.
Systems which operate according to a process of this type are known in coating technology. As an example, see U.S. Pat. No. 5,302,208, although it does not have reactive vapor deposition as its object. In the system described in this publication, an electron beam is used to vaporize a metal oxide. The vapor cloud is converted to a plasma by microwave energy, which is supplied by way of a horn antenna, as a result of which layers with especially good properties are obtained.
For an example of the reactive vaporization of a metal for the coating of a substrate, see U.S. Pat. No. 4,777,062. Like the preceding patent, this document does not deal with the problem of controlling the vaporization rate and thus not with the problem of producing a layer of precisely defined thickness.
For the coating of films with aluminum, it is known that several optical sensors can be provided in the receiver, behind the coating window, distributed across the width of the film. These sensors measure the transmission or optical density of the film coated with aluminum, and the power and/or motion of the electron beam over the metal crucible are controlled in accordance with output of these sensors. When a film is coated with a transparent material such as Al.sub.2 O.sub.3, it is not possible to monitor the thickness of the layer in this way if the coated material is highly transparent. Such transparency is required, for example, in the case of films used to package food products, because it must be possible to see the food products easily, because the color of the food products may not be negatively affected by the packaging, and finally because it must be possible to heat quickly the contents of package in its packaging by means of microwaves. So far, the thickness of transparent layers has been measured by means of x-rays, but this is a relatively complicated method.